Anthropogenic activities, particularly the excessive usage of fossil fuels, have led to a significant increase in the level of CO₂ in the atmosphere since the beginning of the industrial period [1]. This rise in CO₂ levels has contributed to a climate crisis, evident from the recent occurrence of extreme weather events such as floods and forest fires. To address these challenges and reduce greenhouse gas emissions, substantial efforts have been directed toward finding ways to convert CO₂ into value-added chemicals or fuels, including methanol (CH₃OH), formate (HCOO⁻)/formic acid (HCOOH), carbon monoxide (CO), syngas (CO and H₂), and ethylene (C₂H₄) [2]. Among the numerous CO₂ conversion approaches such as biochemical, thermochemical, photochemical, and electrochemical methods, electrocatalytic CO₂ reduction reaction (CO₂RR) is attracting significant attention as an environmentally friendly approach. CO₂RR can be coupled with intermittent renewable energies due to its mild operating conditions with room temperature and atmospheric pressure. It enables the conversion of CO₂ into value-added products without additional emissions (Figure 1) ^[3][4][5][6][3,4,5,6]. Due to their high selectivity (>90%) and activity (>100 mA cm⁻²) for industrial applications, the cost of certain CO₂RR products, particularly C₁ chemicals such as CO and formic acid, has become economically competitive with traditional chemical processes ^{[7][8][9][10][11][12]}[7,8,9,10,11,12]. This trend has been further supported by the decreasing price of renewable electricity.

Liquid products such as HCOO⁻/HCOOH offer significant advantages over gas-phase CO₂RR products due to their high energy densities and ease of storage and distribution ^[13][14][13,14]. Formic acid, in particular, is a crucial chemical feedstock with a global production of approximately 800,000 metric tons per year, used in a wide range of applications, including chemical production, cleaning, the textile industry, and antiseptics ^{[15][16][17][18]}[15,16,17,18]. Moreover, formic acid has gained attention as a promising hydrogen carrier due to its high volumetric hydrogen density (53 g H₂ per liter of HCOOH), low toxicity, and the fact that it is in the liquid phase under ambient conditions ^[19][20][19,20]. It is also considered an energy carrier for fuel cell applications due to its stability, low volatility, and high volumetric capacity ^[21][22][21,22]. Thus, producing formic acid through electrochemical CO₂RR can serve as a carbon-neutral liquid fuel, effectively offsetting carbon emissions [23]. However, several challenges must be addressed for the practical applications of CO₂RR in renewable formic acid production: (i) improving the selectivity, activity, and durability of the catalysts; (ii) reducing the energy consumption and cost of the process; (iii) scaling up the technology and integrating it with renewable energy sources; (iv) separating the product from aqueous mixture. Additional purification steps are often required, increasing production costs ^{[23][24][25][26]}[23,24,25,26].

To address the limitations in CO₂RR for formic acid production, extensive research efforts have been focused on developing efficient electrocatalysts, suitable electrolytes, and reactor designs to achieve high productivity. In particular, catalyst development has received significant attention to improving the selectivity and activity toward HCOO⁻/HCOOH production. Recent studies have made notable progress in identifying and designing efficient electrocatalysts for the selective production of HCOO⁻/HCOOH. For example, studies on the development of nanostructured and bimetallic catalysts have shown promising results in achieving high selectivity and activity for CO₂RR to HCOO⁻/HCOOH ^{[27][28][29][30][31][32][33][34][35][36]}[27,28,29,30,31,32,33,34,35,36]. In addition, single-atom catalysts (SACs) for CO₂RR to HCOO⁻/HCOOH have been investigated due to their high surface area and atomic-level dispersion of active sites, which can enhance their electrocatalytic performance [12].

In early CO₂RR studies, numerous research groups have utilized reactors such as 3-electrode and H-type cells to produce formate in aqueous solutions. This type of reactor has been widely employed for investigating various CO₂RR catalysts due to its versatile setup. In these experiments, the catholyte is saturated with CO₂ by continuous bubbling using a glass sparger before electrolysis. The CO₂ gas is supplied continuously to the catholyte throughout the experiment to improve the absorption of the CO₂ gas in the aqueous solution [27]. Researchers have utilized this reactor configuration to develop highly active and selective electrocatalysts for CO₂RR ^{[27][29][30][31][32][33][34][35][36][37]}[27,29,30,31,32,33,34,35,36,40]. Since the previous works by Hori et al. described the classification of metal catalysts according to product selectivity for CO₂RR ^[38][41], extensive efforts have been made to develop various heterogeneous metal-based catalysts for selective CO₂RR to formate. Table 1 summarizes some of the metal catalysts that have been investigated for their potential in CO₂RR to formate, including tin (Sn), lead (Pb), bismuth (Bi), and indium (In).

Reactor Type	Cathode	Electrolyte (Catholyte)	Potential (V vs. RHE)	FE	formate	(%)	j	formate	(mA cm	−2	)	Reference
3-electrode	Sn/SnO	2	porous hollow fiber	0.1 M KHCO	3	−0.95	82.1	22.9	[32]
	Bi on Cu foil	0.5 M KHCO	3	−0.86	91.3	3.08	[33]
	Bi-Sn	0.1 M KHCO	3	−1.0	93.9	9.3	[34]
H-type	Sn-Pb alloy	0.5 M KHCO	3	−1.36	79.8	45.7	[27]
	SnO	2	/ZnO hollow nanofiber	0.1 M KHCO	3	−1.34	97.9	24.9	[29]
	Pd-Sn alloy/C	0.5 M KHCO	3	−0.43	99.6	~2	[35]
	Mesoporous SnO	2	0.1 M KHCO	3	−1.15	75	10.8	[37]	[40]
	In	16	Bi	84	nano-sphere	0.5 M KHCO	3	−0.94	100	14.08	[30]
	AgSn/SnOx (core-shell)	0.5 M KHCO	3	−0.8	80	16	[36]
	In-doped SnO	2	nanowires	0.5 M KHCO	3	−1.04	82	6.02	[31]

Choi et al. studied the CO₂RR to formate in an aqueous solution using tin–lead (Sn–Pb) alloys [27]. The study found that the Sn_56.3Pb_43.7 alloy exhibits high faradaic efficiency (FE_formate of 79.8%) and current density (j_formate of 45.7 mA cm⁻²) for formate synthesis; 16%, and 25% improved FE_formate and j_formate compared to the single metal electrodes.

Tan et al. reported a highly efficient CO₂RR to formate on a tin (IV) oxide/zinc oxide (SnO₂/ZnO) composite electrocatalyst with a grainy hollow nanofiber (HNF) structure [29]. The faradaic efficiency (FE) of formate on the SnO₂/ZnO composite HNF reaches as high as 97.9% at −1.34 V (vs. RHE), outperforming many Sn-based catalysts. At −1.54 V (vs. RHE), the SnO₂/ZnO HNF exhibits 2 times and 4 times higher current density for formate generation than those of SnO₂ HNF and nanoparticles (NPs), respectively. This superior catalytic performance is attributed to its one-dimensional continuous structure as well as to the synergistic effects between SnO₂ and ZnO, which facilitate faster electron transfer and improve the conductivity of SnO₂/ZnO composite HNF.

Despite these significant results, the kinetics of CO₂RR into formate is limited below 50 mA cm⁻² (Table 1) due to the polarization losses caused by the low solubility of CO₂ in the electrolyte, and the mass transfer limitation of OH⁻ near the electrodes, inevitably limiting the productivity of CO₂RR ^[39][42]. In particular, the low solubility of CO₂ in the aqueous catholyte significantly limits the current density, thus hindering the large-scale application of the CO₂RR.

There have been experiments in CO₂ electroreduction that involve supplying gaseous CO₂ to the cathode, allowing for sufficient CO₂ delivery to the catalyst even at elevated current densities. Some research groups achieved remarkable improvement in the current density of CO₂RR to formate using gas diffusion electrodes (GDEs) typically composed of a backing layer, a microporous layer (MPL), and a catalyst layer (CL) ^{[39][40][41][42]}[42,43,44,45]. Several research groups have successfully utilized GDE-type CO₂RR reactors (Figure 2a) and achieved high current densities for formate production, exceeding 200 mA cm⁻² and even up to 1000 mA cm⁻², with faradaic efficiencies (FE) higher than 90% as summarized in Table 2 ^{[43][44][45][46][47][48][49][50]}[46,47,48,49,50,51,52,53]. However, one challenge with GDE-type electrolyzers is that they typically use aqueous electrolytes, such as KHCO₃, K₂CO₃, or KOH, for the collection of liquid products. The presence of these aqueous electrolytes can dilute the liquid products, such as formate/formic acid, which increases the costs associated with product recovery and separation ^[51][54].

Reactor Type	Cathode	Membrane	Cathodic Potential/Cell Voltage (V)	FE	formate	(%)	j	formate	(mA cm	−2	)	Concentration of HCOO	−	(wt%)	Reference
GDE-catholyte	Bi	2	O	2	CO	3	nanosheet	Fumasep FAB-PK-130	−1.55 (vs. RHE)	93	930	-	[43]	[46]
	InP colloidal quantum dots	AEM	−2.6 (vs. RHE)	93	930	-	[44]	[47]
	SnO	2	nanosheet	Selemion	−1.13 (vs. RHE)	94.2	471	-	[45]	[48]
	Pb powder	-	4	90	352	-	[46]	[49]
	Bi nanosheet	Nafion	®	117	−1.18 (vs. RHE)/3.1	92.4	83.2	0.204	[48]	[51]
	ZnIn	2	S	4	nanosheet	Nafion	®	212	−1.2 (vs. RHE)	94	245	-	[49]	[52]
	Bi nanotube	Selemion	−0.56 (vs. RHE)	98	170	-	[50]	[53]
	Sn/C	Nafion	®	324	−2.44 (vs. SHE)	98	200	-	[47]	[50]
Zero-gap	Bi/C	Nafion	®	117	3.1	71.7	32.3	3.39	[52]	[55]
	Bi/C	Nafion	®	117	3.0	89.2	40.1	33.7	[53]	[56]
	Sn nanoparticle	Nafion	®	115	2.2	93.3	51.7	4.15	[51]	[54]
	Sn nanoparticle	Nafion	®	115	2.2	77.7	29.8	11.62	[51]	[54]
	Bi/C	Sustainion	TM	3.6	91.6	274.8	0.67	[54]	[57]

To overcome this challenge of GDE-type reactors, some research groups have studied CO₂RR to formate in a zero-gap type electrolyzer, which eliminates the need for an aqueous electrolyte ^{[51][52][53][54]}[54,55,56,57]. Lee et al. reported a facile strategy of catholyte-free electrocatalytic CO₂ reduction (CF-CO₂R) that avoids the solubility limitation by supplying an appropriate amount of water vapor with gaseous CO₂ as a cathode reactant ^[51][54]. In this electrolyzer, the water vapor. In the CF-CO₂R electrolyzer, water vapor acts as a carrier for supplying dissolved CO₂ to the cathode by forming a CO₂-saturated aqueous film on the catalyst surface as shown in Figure 2b. The advantage of this approach is that the consumed CO₂ in the film can be immediately replenished from the bulk gas stream, enhancing the mass transfer of CO₂ and improving the reaction kinetics. Lee et al. achieved a formate current density (j_formate) of 51.7 mA cm⁻² at a low cell voltage of 2.2 V using this CF-CO₂R approach. They also obtained a significantly high concentration of formate solution, reaching 116.2 g-formate L⁻¹, with a formate faradaic efficiency (FE_formate) of 77.7% by reducing the vapor supply from 12.58 to 0.65 mg min⁻¹ cm⁻² at 2.2 V and 323 K.

The zero-gap electrolyzer has several advantages over the GDE-catholyte method, including (1) lower cell voltage and improved energy efficiency by eliminating a large ohmic resistance of the catholyte compartment; (2) reduced cost by removing the cost of the catholyte; and (3) zero-gap configuration, which makes the system simple and easy to scale up. However, the lower current density observed in this configuration is indeed due to the challenges associated with the discharge of liquid products and the formation of solid precipitates.

In the zero-gap type electrolyzer, the liquid products have to be discharged in the opposite direction of the CO₂ supply, and ions (HCOO⁻_, HCO₃⁻, CO₃²⁻, K⁺, and Na⁺, etc.) in the liquid products form solid precipitates, which limits the supply of CO₂. Additionally, the flow of aqueous liquid products through the gas diffusion electrode (GDE) can cause a decrease in the hydrophobicity of the electrode over time, a phenomenon known as electro-wetting. This leads to flooding of GDE that significantly inhibits the migration of CO₂ to the catalyst layer, reducing the selective reduction of CO₂ to formates while promoting the hydrogen evolution reaction (HER).

Recently, Díaz-Sainz et al. achieved a high j_formate of 300 mA cm⁻² by continuous operation using an anion exchange membrane (AEM, Sustainion^TM, Dioxide Materials) with a formate concentration of 6.7 g L⁻¹ at a cell potential of 3.6 V ^[54][57]. In this configuration, the formate solution could be obtained at the anode because the HCOO⁻ ions generated in the cathode are transferred to the anode side via the AEM. They even obtained higher j_formate up to 600 mA cm⁻² with a formate concentration of 10.8 g L⁻¹ at a cell potential of 4.7 V. When AEM is used in a zero-gap type electrolyzer for formate production, the cathodic GDE flooding could be significantly reduced because HCOO⁻ moves to the anode through AEM, and the crossover of water and the migration of cations (K⁺, Na⁺, etc.) from the anode is almost completely prevented.